Restaurant, Menu, Order classes in LLD - System Design Exercise

Design: Restaurant Ordering System

Design classes and their relationships for restaurant, menu, and order management. Exclude payment processing and delivery logistics.

Functional Requirements

FR1: Allow creation and management of restaurants

FR2: Each restaurant has a menu with multiple items

FR3: Customers can place orders selecting items from the menu

FR4: Track order status (e.g., placed, preparing, served)

FR5: Support multiple orders per restaurant

Non-Functional Requirements

NFR1: Support up to 100 restaurants

NFR2: Each restaurant can have up to 200 menu items

NFR3: Handle up to 1000 concurrent orders

NFR4: Order status updates should reflect within 1 second

NFR5: System availability target 99.9%

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Restaurant class to hold restaurant details

Menu class to hold menu items

MenuItem class for individual dishes

Order class to represent customer orders

OrderItem class to represent items within an order

Design Patterns

Composition pattern for Menu containing MenuItems

State pattern for Order status management

Observer pattern if real-time order status updates are needed

Reference Architecture

RestaurantOrderingSystem
  |
  +-- Restaurant
        |
        +-- Menu
              |
              +-- MenuItem
  |
  +-- Order
        |
        +-- OrderItem

Components

Restaurant

Class

Represents a restaurant with details and a menu

Menu

Class

Holds a list of MenuItems for a restaurant

MenuItem

Class

Represents a single dish or item available in the menu

Order

Class

Represents a customer's order with multiple OrderItems and status

OrderItem

Class

Represents a MenuItem selected in an order with quantity

Request Flow

1. Customer views Restaurant and its Menu

2. Customer selects MenuItems and quantities to create an Order

3. Order is created with OrderItems referencing MenuItems

4. Order status is set to 'placed' and saved

5. Kitchen updates Order status as it progresses (preparing, served)

6. Customer or system queries Order status for updates

Database Schema

Entities: - Restaurant(id PK, name, address, phone) - Menu(id PK, restaurant_id FK) - MenuItem(id PK, menu_id FK, name, description, price) - Order(id PK, restaurant_id FK, status, created_at) - OrderItem(id PK, order_id FK, menu_item_id FK, quantity) Relationships: - Restaurant 1:N Menu - Menu 1:N MenuItem - Restaurant 1:N Order - Order 1:N OrderItem - OrderItem N:1 MenuItem

Scaling Discussion

Bottlenecks

Database load with many concurrent orders and menu queries

Order status update latency under high load

Memory usage if all menus and orders are cached in memory

Solutions

Use read replicas and caching (e.g., Redis) for menu data to reduce DB load

Implement asynchronous order status updates with message queues

Paginate order queries and limit in-memory caching to active restaurants/orders

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing classes and relationships, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain clear separation of concerns between Restaurant, Menu, and Order classes

Discuss how composition models real-world relationships

Highlight how order status management supports real-time updates

Mention scaling strategies for read-heavy menu data and write-heavy orders

Show awareness of trade-offs in caching and consistency